There are many methods for producing a single crystal. Among them is a CZ method that relates to the growth of a silicon single crystal and that is widely used with its industrially mass-producible system. According to this method, a single crystal is produced first by bringing a seed crystal into contact with the surface of a melt of a crystal material contained in a crucible and then by rotating the crucible and pulling the seed crystal upward while rotating the crystal in a direction opposite to the direction of rotation of the crucible, so that the single crystal grows with the melt solidified at the lower end of the seed crystal.
In producing a single crystal using the CZ method, it is required to completely eliminate dislocations caused by thermal shock at the time the seed crystal is brought into contact with the melt and to thereby rid the dislocations of the body of the single crystal. Usually, a so-called "dash neck process" is adopted as a measure to meet the above requirement. In this process, the diameter of a single crystal is constricted to a small value in order to remove dislocations from the crystal surface to obtain a single crystal free from dislocations. The diameter of a dash neck required for the removal of dislocations is about 3 mm, and its length 30 mm.
The weight of a single crystal produced by the CZ method has heretofore been limited to about 20 to 30 kg. However, there is a strong demand for increased efficiency in the manufacture of semiconductors in recent years, and this has led to a tendency toward the increased diameter and length of a single crystal. As a result, single crystals as heavy as 100 kg or more are often produced. The weight of a single crystal is borne by the slender reduced dash neck portion. However, as the weight of a single crystal to be produced increases as described above, a limit is imposed on the bearing capacity of the dash neck portion. If a single crystal is so heavy that it receives a torsional force or a bending stress during the pulling operation, its dash neck portion is broken and the single crystal falls into the melt in the crucible. In addition, not only inconvenience including the damaging of the pulling apparatus, the overflowing of the melt and the vapor explosions occurs, but also there may be accidents resulting in injury or death.
For the purpose of preventing accidents including the falling of single crystals during the pulling operation which are likely to occur frequently as single crystals become increasingly heavier, improved pulling apparatuses have been proposed (see, e.g., JP3-285893, A and JP3-295893, A). That is, the falling of a single crystal during the course of the pulling is prevented by adopting a structure in which an engaging stepped portion is formed on an upper portion of the single crystal and in which the engaging stepped portion is engaged and gripped by a plurality of catches or a plurality of gripping holders each having a catch. Thus, when the proposed pulling apparatuses having such structure are used, a single crystal, even if it is heavy, can be pulled up reliably as well as safely, according to the disclosure.
The pulling apparatuses, disclosed in JP3-285893, A and JP3-295893, A cause the catches or the gripping holders having catches to engage and grip the engaging stepped portion formed on a single crystal as described above, and with this type of apparatus, it is important, in terms of quality, to eliminate displacement of the center of a single crystal from the pulling axis during the pulling operation. Thus, the aforementioned pulling apparatuses comprise a plurality of catches or a plurality of gripping holders, so that the displacement can be eliminated by finely adjusting the individual catches or gripping holders, and thus the engaging and gripping of the single crystal at the engaging stepped portion is made reliable.
However, since a single crystal is pulled up in a closed vacuum chamber, it is extremely difficult to finely adjust the catches or the gripping holders from outside through a remote operation, in such a manner that the catches or the gripping holders can coincide with the engaging stepped portion of the single crystal that is continuously elevating and rotating at all times. As a result, the adjusting of their positions at which to engage and grip the engaging stepped portion is susceptible to error, and once a displacement occurs, not only a significant impairment in the quality of a pulled single crystal results, but also the single crystal is so greatly inclined by receiving a torsional force during the pulling that the catches or the gripping holders may disengage and thus fall the single crystal.